1. Field of Invention
The present invention relates to chemical analysis systems and techniques. In one example, the present invention relates to methods and apparatus for providing microfluidic electrochemical detection systems with increased sensitivity and reliability.
2. Description of Related Art
The miniaturization of chemical and biochemical analysis instrumentation has enabled the widespread use of such instrumentation in high-throughput and point-of-care applications. High-speed electrophoretic separations of biomolecules are now routinely performed in micron-sized capillaries and have been proven on microfabricated capillary electrophoresis (CE) devices. These techniques offer a significant increase in speed over previous techniques. However, the fluorescence-based detection methods often employed require large and expensive optical detection systems. Significant cost and space savings can be achieved by using electrochemical (EC) methods for detection.
Microfabricated devices are ideally suited to EC detection of biomolecules because of the relative ease of integration of the metallic components required for the detection electrodes. Although conductimetry, voltammetry, and potentiometry have all been employed in CE-EC systems, amperometry is the most widely used EC detection technique for chip-based separations. An important issue when performing amperometric detection with CE is the necessary electrical isolation of the EC detector from the separation voltage, since interference from the CE current significantly influences detector performance. The need to isolate or decouple the CE and EC systems arises from the fact that the separation voltages used in CE typically generate μA-level background EC currents, masking the pA to nA currents measured at the working electrode of an EC detection system. Therefore, for optimum CE-EC performance, the electrical overlap of the two systems must be controlled and minimized.
Several approaches have been developed to minimize the influence of CE fields, including end-channel detection on chip, end-channel detection off-chip, in-channel detection and off-channel detection. In- and off-channel detection involve the placement of the working electrode directly within the separation channel using either an electrically isolated “floating” potentiostat or electric-field decouplers. End-channel (or end-column) detection is the most commonly used mode of amperometric detection, where the working electrode is placed close to, for example, 10 to 50 microns (μm), the exit of the separation channel. The majority of the CE voltage is dropped along the high-resistance capillary or separation channel (inside diameter, for example, approximately 25 μm) resulting in very little of it being dropped in the low-resistance detection reservoir, minimizing current in the detector region. However, since the grounded-end of the CE circuit is within the detection reservoir, the remaining electric field causes potential shifts at the working electrode of the detector. In general, end-channel detection exhibits high background currents and separation efficiency is sacrificed due to diffusion of the analyte in the region between the separation channel end and the working electrode of the EC detector.
Therefore, it is desirable to reduce background currents and increase the sensitivity of EC detectors. It is also desirable to enhance the separation efficiency of CE systems.